Title of volcano disaster movie of ca. 1975

Hi there, this one is for the movie buffs and especially for those who like disaster movies. I watched a volcano disaster movie with my parents back in around 1975 when I was about 6 years old (I think the latest date I can place it, is around 1977 or so). I cannot remember the title and therefore I will have to describe some of the scenes in the movie. The parts I remember most vividly revolve around some kind of research centre or laboratory perched on the mouth of a volcano. I believe this building was kind of whitish, looking from the outside. In one particular scene three men go down in a kind of elevator taking them deeper inside the volcano. The floor of the elevator was made of thick glass and, not surprisingly, at some point the glass cracked and one of the men nearly fell out of the elevator. He was rescued just in time by the other men in the elevator. Around the end of the movie the research centre falls into the volcano when it starts to erupt, I recall one scene where the scientists run around in the main control room, panicking, when this happens. I cannot recall if there were other points of interest in the movie like a town being destroyed as in other similar movies. I hope someone out there can help here, it has tickled me for ages trying to find out what this movie's title is.